Caramelize onion in large cast iron skillet. Remove and set aside.
Add some butter and fry bread slices until toasty (absorbs the caramelized onion flavor, just trust me on this one)
If using leftover caramelized onions, add butter to skillet and reheat then complete above step
Add more butter and fry lentil loaf until slightly browned on sides
Mix ketchup, mayo, and mustards, adjusting proportions to taste.
Smear condiment mixture on inside of all pieces of bread. On left piece of bread, add caramelized onions, then lentil loaf, then pickles followed by second piece of bread. Repeat.